The present invention relates to a head suspension assembly in a disc drive, such as a hard drive using a magnetic storage medium. More particularly, the present invention relates to a disc drive head suspension assembly using a damping material to reduce high frequency vibration.
Disc drives are one of the key components to store data in a computer system. In a basic hard disc drive, data is stored in a magnetic medium formed on a surface of a rotating disc. The hard disc drive reads and writes information stored on tracks on a disc bearing the magnetic medium. To do this, a read/write head that includes a transducer carried by a slider assembly is placed in close proximity to the surface of the magnetic medium; The slider is attached through a gimbal system to a distal end of head suspension which, includes the head suspension. The proximal end of the head suspension is attached to an actuator arm which is rotatably controlled by a voice coil motor (VCM). The disc drive system sends control signals to the voice coil motor to move the actuator arm and the suspension supporting the read/write head across the disc in a radial direction to the target track. The positioning of the read/write head over the magnetic medium is controlled by a closed, loop circuit for better accuracy. In addition to the active controlling signal from the closed loop circuit, the precise positioning of the read/write head is affected by a dynamic balance between two vertical forces. The first force is a gram load applied by the head suspension to bias the head toward the disc surface. The second force is an air bearing lifting force caused by the fast motion between the slider and the disc surface. Roughly, the control system controls tracking (i.e., radial positioning of the head) while the dynamic balance determines fly-height (i.e., head-media spacing). However, as the areal density of concentric data tracks on magnetic discs continues to increase (that is, the size of data tracks and radial spacing between data tracks decrease), hard disc systems also use active control for more precise vertical positioning of the head.
One of the most significant adversarial conditions affecting precise positioning of the read/write head in a disc drive system is vibration, particularly that caused by head suspension resonance. Many types of vibration exist in a disc drive system to cause fluctuation of the magnetic read/write head positioning. In particular, vibrations resulting from resonances of the system are often serious obstacles in improving areal density and rotation speed of the disc drive system. Every closed loop servomotor system has a bandwidth, and resonances occurring within the bandwidth degrade the performance of the servomotor system. In a hard disc drive, for example, windage excitation (fluid turbulence caused by airflow) can cause head vibration at resonance frequencies of the head suspension assembly and thus cause the head suspension assembly to have large displacement amplitudes. Windage, however, is not the only source that can cause resonance in a hard disc drive system. In today's high-speed hard disc drives, the servomotor that moves the parts at high frequency may also cause resonance. In addition, when it is desired to position the magnetic head to a specific track location, the voice coil motor is driven by a voltage that has a very short rise time to accelerate the actuator very quickly. Once the actuator is in motion, the voltage levels off and the actuator approaches a constant velocity. As the actuator approaches the target location on the disc, a similar, but inverse abrupt voltage pattern is applied to the voice coil motor to stop the suspension actuator. This sequence of voltage change is best represented by a square wave, which is a superposition of many waves of different frequencies, according to Fourier transform. The operation of the servo system in a hard disc drive to move the suspension head assembly thus has inherent frequency components that may excite resonance.
Resonance degrades the performance of a disc drive in several ways. First, severe resonance, especially that of sway or torsion mode, may cause the magnetic read/write head to move away from the target track and thus result in data reading/writing error. Second, resonance in the vertical direction, such as that caused by resonance in bending mode, may cause fluctuations in the fly height of the read/write head to result in data error as well. In extreme cases, vertical fluctuations may even cause catastrophic damage of the disc drive due to direct contact between the head and the disc surface. Third, during resonance, the transducer element of the read/write head is forced to modulate, causing a significant decrease in the signal to noise ratio of the system and increase of the non-repeatable run-out (NRRO).
Significant efforts have been made to alleviate the problem of resonance. Various methods have been used to address the problem of resonance. The product design is essentially an optimization of the system involving a balance of several factors, often gaining on one aspect at a cost of sacrificing another, as commonly found for a spring-mass-damper system.
With the increasing demand for disc drives that are more reliable, quieter and faster, and have larger storage capacity (with increased areal density) and smaller overall disc size, there is an increasing need for a disc drive suspension system having better balanced optimization between several performance properties including damping property, stiffness and the structural integrity.